Output Voltage of Colpitts Oscillator

[thanhFF]

Hi all,

I am doing simulation for simple Colpitss Oscillator, the circuit taken from Thomas Lee book( Chapter 17, page 623). I am using Cadence Virtuoso and SpectreRF to test that. The estimation output frequency was similar to the one discribed in the book (around 60Mhz). However, I dunno how can I get the output voltage based on the equation there. They calculated the output voltage is about 1.4V. But the output voltage i had is very very small.
The NMOS have L = 0.35u, and I changed W based on the number of fingers
With W = 80u, W/L = 228, the f = 64Mhz, Vout(p-p) = 1.2 uV
With W= 1600u W/L= 4570 the f = 56Mhz, Vout(p-p)= 0.55mv

I change the number of fingers from small to large, to see Vout change, but they are still ver small. In the page 624 they recommended choose W/L =6000 to ensure the start-up condition. I also tried that but the Vout sitll was small.

So my question is why my Vout was so small, I read some equations, but be honest dont understand much, Is it Vout is not related to Vbias and Vdd at all ( in the book Vout just only depends on Ibias,R and capacitivite divide factor)
What I can do to make Vout bigger?
I attached the schematic and result for two cases above


Thanks:-D

 

[thanhFF]

Forgot to mention Vdd = 3V, and Vbias = 1.5V

 

[FvM]

It seems unlikely, that the circuit is able to maintain this low steady-state voltage. I rather expect an error in steady-state determination. Did you try a regular transient simulation?

 

[vfone]

Increase the idc to 10mA and it will work.

 

[thanhFF]

Tks vfone, and fvM. I did change the idc to 10mA, and the voltage increased. But why do we have this chane? Can you explain little bit more for me please?
And if i want to get exactly 1V(p-p), wat should I change, are there any equations to calculate this

 

[vfone]

Read this to understand what is cut-off, linear and saturation regions of the MOSFET:
https://ocw.mit.edu/courses/electri...nd-circuits-fall-2005/lecture-notes/lec10.pdf

Now, changing in small limits the Vbias can change the output level.

This you should do before asking any questions. Play with currents and voltages and see what happen, because using a simulator you cannot burn anything.

 

[thanhFF]

Hi vfone. So in the circuit above, the oscillator is running in saturation region, isnt it? I read in some IEEE papers, they said its very hard to calculate amplitude for Vout. I am really confused now.

I tried with Idc=10ma, W/L = 2285, the Vout = 4.8x2(p-p) ( I think the oscillator in cut-off region, but not sure)
but with W/L = 1828 the Vout just around 25mvx2(p-p)

Can you show me the very simple way to calculate or adjust the output voltage. I still dont understand why we change Idc =10ma, the output increased(too much indeeed). I wanna get 1V output voltage.

Tks

---------- Post added at 03:41 ---------- Previous post was at 03:40 ----------

Also, play with Vbias around 1.5 to 1V. The result wasnt much different